Accompanied by a translator and a missionary who works with children on the streets, I spoke to three very young girls, one of whom was clearly not well because of the drugs she was taking and glue she was sniffing.

Lorrisa is only 13 and her frame so tiny that I am sure her growth had been stunted by drugs. She sniffed from a plastic bottle containing strong industrial glue, which they ­nickname “cola”, as she explained how she lives.

I held the bottle under my nose to see what it was like and the smell was overpowering, leaving me with a ­splitting headache. Lorrisa told me: “Sniffing the glue makes me feel dizzy and numb and it stops me feeling hungry so I don’t need to eat. It helps me cope with the ­violence and danger on the streets.”

I was joined by Lorrisa’s friend Raphaela, 13. She told me how she gets approached by scores of men looking for sex – locals, tourists and taxi ­drivers. They will pay 10 Brazilian real, the equivalent of £2.60.

She recounted a frightening story about her friend Mickela, 14. She said: “Mickela lived on the street and she was murdered on the dayof a police strike. A man picked her up by the Metro train station and she had sex with him. But afterwards he refused to pay, killed her and dumped her body. It only ­happened a few weeks ago.”

In the three hours I spent with the children as they openly went about their business, a police car passed twice but officers did nothing. Pimps come and go alongside drug dealers, safe in the knowledge the police are unable to stem the horror.

Calliem, 14, told me how she first started selling her body for sex at 11. “I have sex so many times with men and they only pay me five Brazilian real,” she said. It is the ­equivalent of £1.30. Calliem is a regular user of “cola”, cannabis and crack cocaine – acquiring the crack from a regular dealer who visits the streets.

Calliem told me she fears the World Cup will just attract more men wanting to have sex. A woman aged 41, who has lived on the streets since she was seven, said: “The children are at real risk from local men and tourists.”

She pointed out one boy aged 10 who has been forced into prostitution. She said: “The children go with the men because they are high on drugs or need more moneyto buy drugs. They use drugs to numb the pain of the sexual abuse, become addicted then need to sell themselves over and over again to raise the money.”

Drugs are widely available in Brazil and the country is in the grip of a crack cocaine epidemic.

The charity Happy Child, set up to care for ­children who get pregnant by selling themselves for sex, has launched a warning campaign for World Cup fans which includes wristbands for the kids.

Chief Executive Sarah de Carvalho MBE said: “We are worried about the influx of fans. We know with ­major sporting events that vulnerable children are at risk of being sexually exploited. Our It’s A Penalty campaign has been set up to raise awareness among football fans that if they engage in sexual ­exploitation with a child aged 17 or under they could face prosecution in Brazil and theirhomecountry.”